The present invention relates to the general field of variable-section flow mixers for double-flow turbojets with a low bypass ratio for supersonic airplanes.
The low bypass ratio double-flow turbojet for a supersonic airplane essentially comprises a double-flow gas generator (generating a cold flow and a hot flow), extended by a nozzle that defines a gas ejection channel.
Low bypass ratio double-flow turbojets for fitting to supersonic airplanes for use in civilian transport need to comply with two requirements: firstly they must present as little drag as possible during stages of transonic and supersonic cruising flight; and secondly they must present acceptable levels of noise on airplane takeoff, with certification authorities having increasingly more stringent requirements with respect to the emission of noise from the turbojets of civil airplanes.
Unfortunately, those two requirements are contradictory. The first requirement leads to turbojet designs of small diameter, while the second requirement needs thrust to be increased by increasing the mass flow rate of gas, thereby leading to large fan diameters (and thus to large turbojet diameters).
One known solution for complying with those requirements is to make use of a variable-section flow mixer. Such a mixer makes it possible during airplane takeoff stages to introduce into the turbojet a flow of air that is external to the turbojet in order to be mixed with the flow of gas coming from the gas generator. The mixer for mixing external air with the gas flow coming from the gas generator serves to increase the total mass flow rate of gas that is produced by the turbojet. Thus, at constant thrust, the speed at which the gas is ejected can be reduced compared with a double-flow turbojet that does not have a mixer. Since jet noise increases with gas ejection speed, this reduction in speed leads to a large decrease in noise level on takeoff.
In practice, the external air is introduced into the turbojet downstream from the gas generator via openings that are distributed around the entire circumference of the nozzle. The air introduced in this way mixes with the gas flow coming from the gas generator with the help of guides that extend radially across the gas flow ejection channel. Those guides are movable between a position in which they disengage the openings and enable mixing to take place (during airplane takeoff stages), and another position in which they obstruct the openings for other stages of flight.
Although satisfactory, that solution presents the drawback of requiring the turbojet to be lengthened considerably in order to encourage mixing between the external air flow and the gas flow coming from the gas generator. Unfortunately, the turbojet can be lengthened only to the detriment of its weight. Furthermore, it is generally necessary to have recourse to acoustic lagging on the inside wall of the ejection nozzle of the turbojet so as to absorb the most troublesome sound frequencies.
A mixer is also known from French patent application No. 06/50127 (published under FR 2,896,274) filed by the Applicant, in which the guides are lobes, all presenting an azimuth component in the same direction so as to impart gyratory motion to the external air admitted into the turbojet when the lobes are in their open position.
Although effective in reducing the jet noise level of turbojet on takeoff in compact manner, that solution nevertheless leads to a loss of thrust due to the gyration of the external air that is introduced into the turbojet when the lobes are in their open position.